Methods of forming fluoropolymer products are known in the art, including extruding polytetrafluoroethylene (PTFE) products. PTFE is commonly ram extruded after being treated with a lubricant and formed into a billet. With extrusion, a “green” PTFE product is formed. As is well known, the lubricant may be volatilized, and the “green” PTFE may be expanded into a fibrillated state (referred to as expanded PTFE or ePTFE) and, thereafter, heated above sintering temperature to coalesce the material into a stable state. FIG. 1 is a micrograph showing a typical fibrillated microstructure of uniaxially expanded PTFE. The elongated dark portions which generally extend in a top-to-bottom direction in the plane of FIG. 1 are nodes. Fibrils are thin hair-like structures which extend left-to-right in the plane of FIG. 1 and interconnect the nodes. As is typical with uniaxially expanded PTFE, the fibrils are disposed generally along the expansion direction.
Techniques have been developed in the prior art to expand PTFE in multiple steps, but along different axes. For example, it has been known to expand PTFE in a first longitudinal direction and then separately in a second perpendicular transverse direction to form biaxially expanded PTFE. U.S. Pat. No. 5,476,589, which issued on Dec. 19, 1995 to Bacino, discloses a method including in sequence: 1. transverse expansion of a PTFE tape; 2. two separate longitudinal expansions of the tape; 3. transverse expansion of the tape once again; and, 4. sintering of the final product.
Additionally, U.S. Pat. No. 5,749,880, which issued on May 12, 1998 to Banas et al. discloses longitudinally expanding PTFE tubes; encasing a stent within the tubes; sintering the tubes; and causing radial expansion of the assembly, which results in nodal deformation in the tubes. The previous multiple expansion techniques, however, have never dealt with obtaining ultra-high uniaxially expanded fluoropolymer, more particularly, ultra-high uniaxially expanded PTFE.